Printed circuit boards consisting of a pattern of wiring on an insulating base or substrate, have become the standard method of construction of practically all electronic systems.
One method of producing printed circuit boards includes a subtractive process wherein a metal foil, usually copper, is bonded to an insulating substrate and then etched to form a required pattern. Another method of producing printed circuit boards includes an additive process wherein metal deposition is used to make the pattern on the surface of an insulating substrate.
The printed circuit boards made by the above-mentioned methods may have conductors on one side of the insulating substrate, commonly known as single-sided, or may have conductors on both sides of the insulating substrate, commonly known as double-sided. In addition, a number of the printed circuit boards made by the above-mentioned method may be stacked and bonded together to form a multiple-layered board. Although these printed circuit boards provide the means for creating complex interconnecting patterns, they lack the capability to effectively extract the heat generated by power devices installed on their surface.
One of the important design considerations in utilizing printed circuit boards in electronic packaging is removal of heat generated by high power dissipating devices which are mounted on the board. Conventional heat removal structures include a strip of metal connected to the printed circuit board as a heat sink layer. The high power device would then be disposed on the heat sink layer and their leads connected to the printed circuit board. A major disadvantage of such heat sink layers is that the total package including the printed circuit board and the heat sink becomes inefficiently large and heavy with low average component density per unit area.
A recent method for providing printed circuits with high thermal performance utilizing a metal substrate printed circuit board is disclosed in U.S. Pat. No. 4,810,563 issued Mar. 7, 1989 to DeGree et. al.
The thermally conductive circuit board is built by laminating a thin layer of high performance insulator on top of a metal substrate and bonding a thin conductive metal layer on the insulator. The conductive layer may be etched to form a desired circuit pattern thus resulting effectively in a printed circuit board.
Successive layers of insulation and conductors can be added to form a multilayered board. The metal substrate of the board may be mounted on a heat sink. Consequently, heat can be removed very efficiently from devices mounted on the printed circuit board.
There are several disadvantages associated with metal substrate printed boards. For instance, the cost of processing the board with a metal substrate layer and an insulation layer is higher than the cost of processing a conventional, insulator based, printed circuit board. Furthermore, electrical components cannot be installed on both sides of the metal substrate, printed circuit boards as opposed to conventional printed circuit boards with double-sided capability. Another disadvantage is that only surface-mounted components can be used in conjunction with metal substrate, printed circuit boards; since through hole mounting of components is impossible. Furthermore, operation of circuits assembled on such a board may be impaired due to the presence of large stray capacitances.